Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aircraft, automotive, and sporting goods applications. Typically they comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a thermosetting resin matrix.
Composite properties depend on both the matrix resin and the reinforcement. In unidirectional carbon fiber composites, important mechanical properties include longitudinal strength and modulus, transverse tensile strength and modulus, and longitudinal compressive strength. The matrix affects all of these properties, but has the greatest effect on compressive strength and transverse tensile properties. High composite compressive strengths and transverse tensile moduli require that the matrix have a high modulus.
State-of-the-art epoxy matrix resin systems in advanced composites are typically based on N,N,N',N'-tetraglycidyl 4,4-diaminophenyl methane and 4,4-diaminodiphenyl sulfone. These resins produce unreinforced castings which have tensile strengths of about 8,000 psi and tensile moduli of 500,000 to 550,000 psi. Unidirectional composites containing 60 volume fraction fiber made with these matrix resins typically have transverse tensile strengths of 5,000 to 7,000 psi and transverse tensile moduli of 1.0 to 1.4 million psi. Higher transverse properties are very desirable for applications such as pressure vessels. Improved compressive properties are desirable for structures subjected to high compressive loads, such as sucker rods for oil wells.
Epoxy resin systems affording higher matrix properties than those in state-of-the-art formulations are known. For example, compositions comprising bis(2,3-epoxycyclopentyl) ether and m-phenylenediamine produce unreinforced castings with superior tensile strengths (e.g. 14 to 17.000 psi) and tensile moduli (650 to 670,000 psi).
Related epoxy compositions are described in U.S. Pat. No. 3,398,102, which discloses tacky, curable copolymers formed by reacting bis(2,3-epoxycyclopentyl) ether with aliphatic polyols such as ethylene glycol. Castings made by curing these compositions with m-phenylenediamine have some of the highest tensile strengths (16 to 18,000 psi) and tensile moduli (700 to 850,000 psi) of any thermosetting material. However, in commercial production, these epoxy compositions require long reaction times and complicated vacuum distillation and water washing steps as part of their production processes. Moreover, the yield of the final resin is low, typically 40 to 50%.
It has now been found that epoxy compositions which produce unreinforced castings with a high level of tensile properties and a high heat deflection temperature can be obtained by blending a cycloaliphatic epoxy resin such as bis(2,3-epoxycyclopentyl) ether with a specific group of modifiers. The blend of cycloaliphatic epoxy resin and modifier produces castings with properties comparable to those obtained with the compositions described in U.S. Pat. No. 3,398,102. This process is simpler and affords the final product in higher yields than that used to make copolymers of bis(2,3-epoxycyclopentyl) ether and aliphatic polyols.
In the prior art, compositions have been prepared with glycidyl type epoxy resins such as bisphenol-A epoxy resins and modifiers which are described as antiplasticizers. Hata et al in Journal of Applied Polymer Science, Volume 17, pages 2173 to 2181 (1973) and Volume 21, pages 1257 to 1266 (1977) disclosed that polychlorinated biphenyls, dibutyl phthalate and the phenol adducts of bisphenol-A epoxy resins are effective antiplasticizers in a bisphenol-A epoxy resin cured with ethylenediamine. Also, Khozin et al in Polymer Science USSR, Volume 21, pages 1938 to 1948 (1980) described modifiers such as polyhalogeneated biphenyls, azobenzene, hydroxy diphenyl, carbon tetrachloride, tricresyl phosphate in bisphenol-A epoxy resins cured with 4,4-diaminodiphenyl methane. P. D. McLean et al in The British Polymer Journal, Volume 15, March 1983, pages 66 to 70 described additives for bisphenol-A epoxy resins cured with 4,4-diaminodiphenyl methane. The additives are described as fortifiers. They are reaction products between a substituted aromatic amine or amide and a monoepoxide or diepoxide.
However, there are no references to the use of any of the above modifiers with cycloaliphatic epoxides. None of the modified aromatic epoxy resin compositions in the prior art show the combination of tensile modulus greater than 680,000 psi, tensile strength above 10,000 psi with a heat deflection temperature above 100.degree. C. The compositions of this invention, when cured with selected aromatic amines, meets all these requirements.